1. Field of the Invention
The present invention relates to a sheet finisher, an image forming apparatus using the same, and a sheet finishing method, particularly to a sheet finisher to perform a folding process of a printed sheet, an image forming apparatus using the same, and a sheet finishing method.
2. Description of the Related Art
Hitherto, there is known a sheet finisher which is placed downstream of an image forming apparatus, such as a copier, a printer or an MFP (Multi-Function Peripheral), and performs a post-processing, such as a punching process or a stitching process, on a printed sheet.
Recently, the function of this sheet finisher is diversified, and a sheet finisher is proposed which has, in addition to the function of the punching process and the stitching process, the function of a folding process to fold a part of a sheet, and the function of a saddle-stitching and folding process to staple the center of a sheet and then to fold the sheet at the center (JP-A 2004-59304, JP-A 2003-182928, etc.)
In the sheet finisher having the function of the saddle-stitching and folding process, it becomes possible to form a booklet (to bind a book) from a plurality of printed sheets.
In the saddle-stitching and folding process proposed hitherto, after the canter of sheets is stitched with staples or the like, a process is performed in which a fold line is formed on the stitched part by a pair of rollers called fold rollers and folding is performed. At this time, a plate-like member called a fold blade is brought into contact with the stitched part of the sheet bundle, and is pressed into a nip of the fold roller pair to form the fold line on the sheet bundle.
However, the time in which the folded part of the sheet bundle is pressed by the nip of the fold rollers is short, and the whole folded part is simultaneously pressed by the nip of the fold rollers, and accordingly, the pressure is dispersed to the whole fold line. Thus, the fold line formed by the fold rollers becomes the fold line to which the pressure is not sufficiently applied. Particularly, in the case where the number of sheets is large, or in the case where a thick sheet is contained in the sheet bundle, the fold line often becomes incomplete.
In order to deal with this problem, JP-A 2004-59304 or JP-A 2003-182928 discloses a technique in which a roller called a reinforce roller is additionally provided, and the fold line formed by the fold rollers is reinforced by this reinforce roller.
In the technique disclosed in JP-A 2004-59304, the sheet bundle pushed out from the fold roller is temporarily stopped on a guide plate, and the reinforce roller is moved along the fold line while applying pressure to the fold line of the sheet bundle from above. The fold line nipped between the guide plate and the reinforce roller is reinforced by the pressure generated between the guide plate and the reinforce roller.
JP-A 2003-182928 also discloses a technique in which a fold line pushed out from a fold roller is nipped in a nip of a pair of reinforce rollers, and the pair of reinforce rollers is moved along the fold line to reinforce the fold line.
Incidentally, in general, the center of a sheet bundle is saddle-stitched with staples, and further, the center is folded and a fold line is formed. In the case where the thickness of the sheet bundle is thin, the thickness of the staple in the fold line portion becomes dominant over the thickness of the sheet bundle itself. With respect to the fold line of the thin sheet bundle as stated above, when a reinforce roller is moved along the fold line to reinforce the fold line, the surface of the reinforce roller is damaged by the staple. When this damage becomes large, the fold line is wrinkled in an area except the position of the staple or the sheet is scratched.